vec2 cart2polar(vec2 uv){
    float phi=atan(uv.y,uv.x);
    float r=length(uv);
    return vec2(phi,r);
}

void mainImage(out vec4 fragColor,in vec2 fragCoord){
    vec2 uv=fragCoord/iResolution.xy;
    uv=(uv-.5)*2.;
    uv.x*=iResolution.x/iResolution.y;
    uv=cart2polar(uv);
    fragColor=vec4(uv,0.,1.);
}
